char string[20] = "aman"; Will allocate an char array with a size of 21 (the extra one, is for the null terminator), giving you [a][m][a][n][0][0][0][0][0]...[0]
Whereas: char string[] = "aman"; Will allocate an char array with a size of 5 (again, extra one, is for the null terminator), giving you [a][m][a][n][0]